Browsing by Author "Teixeira, P. Alexandra"
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- Environmentally friendly matrix effect matching evaluation for the major elements in solid biofuel characterization by atomic absorption spectrometryPublication . Trancoso, Maria Ascensão; Lopes, Helena; Teixeira, P. Alexandra; Calisto, Sandra C.The thermo-chemical conversion of biomass has become increasingly important, from the last decade, due to the need to reduce CO2 emissions and to increase renewable energy sources. Currently, the use of biomass already has a strong economic environmental and social impact. The biomass source can be very different where by biomass characterization is a crucial task allowing the prediction and prevention of problemas associated with biofuel burning.
- Evaluation of acid matrix effects in the determination of major elements in biomass by atomic absorption spectrometry from an environmentally friendly point of viewPublication . Teixeira, P. Alexandra; Calisto, Sandra C.; Lopes, Helena; Trancoso, Maria AscensãoThe estimation of major element content in solid biofuels is required for prediction and prevention of eventual ash-related problems during combustion. These analyses have to be achieved with minimum impact on the environment. The quantitation of Al, Ca, Mg, Na, K, Fe, Si and Mn in biofuels was carried out according to EN 15290 using acid decomposition of solid samples followed by atomic absorption spectrometry (AAS). A microwave-assisted acid digestion with a HNO3/H2O2/HF mixture was used, followed by HF complexation using H3BO3. Due to the presence of tetrafluoroboric acid complex in the digestion solution, matrix effects were noticed during elemental quantification by AAS. Standard addition calibration methods did not compensate for this matrix effect. Matrix effects that constrain an analytical response may be overcome by applying the procedure used for samples to the calibration standards using the same reagents. However, this entails using large amounts of toxic reagents. In this work, the fluoric–boric acid matrix matching was assessed statistically using one-way ANOVA tests. For the seven groups of nitric acid and reagent blank (HNO3/H2O2/HF/H3BO3) mixtures used, ranging from 0 to 1 volume ratios, no significant differences were observed for Si, Al, Fe and Mn. The calculated F values were lower than the critical value, F 6,14 = 2.85 (p = 0.05). However, for Ca, Mg, Na and K, significant differences were observed. Tenfold dilution was used for samples where the mass fraction exceeded the analytical dynamic range of the AAS instrument. The calibration solutions were prepared using the reagent blanks in the same proportion thus decreasing the amount of acids used. The procedure was validated using SRM 1573a—tomato leaves—purchased from the National Institute of Standards and Technology. Target recoveries of (1 ± 0.1) were achieved.
- Evaluation of slagging and fouling tendency during biomass co-firing with coal in a fluidized bedPublication . Teixeira, P. Alexandra; Lopes, Helena; Gulyurtlu, Ibrahim; Lapa, Nuno; Abelha, PedroOver the last decades, several indices based on ash chemistry and ash fusibility have been used to predict the ash behaviour during coal combustion, namely, its tendency for slagging and fouling. However, due to the physicalechemical differences between coals and biomass, in this work only the applicability of an ash fusibility index (AFI) to the combustion and co-combustion of three types of biomass (straw pellets, olive cake and wood pellets) with coals was evaluated. The AFI values were compared with the behaviour of ash during combustion in a pilot fluidized bed and a close agreement was observed between them. For a better understanding of the mechanisms associated with bed ash sintering, they were evaluated by SEM/EDS and the elements present on the melted ash were identified. Evidences of different sintering mechanisms were found out for the fruit biomass and herbaceous biomass tested, depending on the relative proportions of problematic elements. The particles deposited on a fouling probe inserted in the FBC were analyzed by XRD and the differences between the compounds identified allowed concluding that the studied biomasses present different tendencies for fouling. Identification of KCl and K2SO4 in the deposits confirmed the higher tendency for fouling of fruit biomass tested rather than wood pellets.
- Measurement of total reactive phosphorus in natural water by molecular spectrophotometry (SMEWW 4500-P D)Publication . Silva, Ricardo Bettencourt da; Trancoso, Maria Ascensão; Teixeira, P. Alexandra; Oliveira, Cristina M. R. Ramiro de; Mosca, Alice I. de Jesus; Dias, Florbela A. do Sacramento; Camões, M. Filomena G. F. CrujoABSTRACT: Phosphorus occurs in natural waters almost solely as phosphates.
- Slagging and fouling during coal and biomass cofiring: chemical equilibrium model applied to FBCPublication . Teixeira, P. Alexandra; Lopes, Helena; Gulyurtlu, Ibrahim; Lapa, Nuno; Abelha, PedroA thermodynamic model was applied to foresee the occurrence of fouling, slagging, and bed agglomeration phenomena during fluidized bed monocombustion of three different types of biomass, namely straw pellets, olive cake, and wood pellets. The cocombustion effect in reducing the occurrence of deposits and agglomerates of blends of 5, 15, and 25% (wt.) biomass with coal was also assessed. Chemical fractionation was applied to evaluate the reactive and nonreactive fraction of elements in the fuels, which was used to estimate their partition between the freeboard and bottom zone of the boiler. Qualitative and semiquantitative analytical techniques, namely, X-ray diffraction and scanning electronic microscopy – energy dispersive spectroscopy were used to compare the results from the simulation with the mineralogical and morphological composition of ash and deposits formed during combustion. The thermodynamic modeling revealed to be a powerful tool in foreseeing the formation of melt and liquids salts, depending on the temperature and chemical composition of fuels. The main discrepancies observed between the experimental and simulated data were due to particularities of the combustion process, which are not incorporated in the software, namely, kinetic limitations of the reactions, possible occurrence of secondary reactions in the ashes, and elutriation effects of ash and silica sand particles.
- Uncertainty estimation to evaluate mass balances on a combustion systemPublication . Teixeira, P. Alexandra; Lopes, Helena; Gulyurtlu, Ibrahim; Lapa, NunoMass balances of ash and potassium for a fluidized bed combustor were performed incorporating measurement uncertainties. The total output mass of ash or a chemical element should be equal to the mass in the input fuel; however, this is not often achieved. A realistic estimation of recovery uncertainty can support the reliability of a mass balance. Estimation of uncertainty helps to establish a reliable evaluation of the recovery ratio of ash mass and elemental mass. This may clarify whether any apparent lack in closing the mass balance can be attributed to uncertainties. The evaluation of measurement uncertainty for different matrices, namely coal, biomass, sand and ashes from different streams was based on internal quality control data and external quality control data, namely analysis of samples from proficiency tests or use of a certified reference material. The evaluation of intermediate precision and trueness allowed the estimation of measurement uncertainty. Due to the different physic and chemical characteristics of the studied matrices, the uncertainty of precision was evaluated using R-charts of data obtained from the analysis of duplicates for the majority of samples. This allowed evaluating sample heterogeneity effects. The instrumental acceptance criterion was also considered and included in the combined uncertainty. The trueness was evaluated using data from several proficiency tests and from analysis of a certified reference material or sample spiking. Statistically significant bias was included.
- Use of chemical fractionation to understand partitioning of biomass ash constituents during co-firing in fluidized bed combustionPublication . Teixeira, P. Alexandra; Lopes, Helena; Gulyurtlu, Ibrahim; Lapa, NunoThree species of biomass origin (straw pellets, olive cake and wood pellets) and two coals from different countries (Coal Polish and Coal Colombian) have been studied to understand the fate of their ash forming matter during the combustion process and to investigate the influence of co-firing biomass with coal. Three different approaches to investigate the ash behaviour were employed: (1) chemical fractionation analysis to evaluate the association/reactivity of ash forming elements in the fuels as a prediction tool, (2) establishment of elements partitioning in ash streams produced in the combustion and co-combustion trials, and (3) evaluation of enrichment factors of elements in the ash streams. The chemical fractionation analysis was applied to all fuels used to evaluate how the association/reactivity of elements making up ash may influence their behaviour during combustion. Combustion tests were carried out on a pilot scale fluidized bed combustor (FBC). Four ash streams were obtained at different locations. The uncertainty of measurements was estimated allowing a critical evaluation of mass balances over the combustion system and the partitioning of elements in the ash streams. The enrichment factors of elements in the several ash streams were estimated, incorporating uncertainties associated with analytical measurements. Results obtained showed that for FBC the relation between the chemical fractionation and the experimental partitioning is strongly affected by elutriation of particles. The element enrichment factor estimated for each ash stream, using Al as a reference element, revealed better correlations with the elements reactivity obtained by chemical fractionation because it overcomes particles elutriation effects. Nevertheless, it was observed that the reactivity estimated by chemical fractionation could not be solely interpreted as tendency of the elements to volatilize on FBC system, as reaction in bed zone of boiler may also occur retaining reactive elements.
- Use of chemical fractionation to understand partitioning of biomass ash constituents during co-firing in gluidized bed combustionPublication . Teixeira, P. Alexandra; Lopes, Helena; Gulyurtlu, Ibrahim; Lapa, Nuno